A potentiometer wire of length \( 10 \, \text{m} \) and resistance \( 20 \, \Omega \) is connected in series with a \( 25 \, \text{V} \) battery and an external resistance \( 30 \, \Omega \). A cell of emf \( E \) in secondary circuit is balanced by \( 250 \, \text{cm} \) long potentiometer wire. The value of \( E \) (in volt) is \( \frac{x}{10} \). The value of \( x \) is ________.
